NEW YORK (Reuters Health) - Combining the results of 17 studies on depression and stroke, researchers found that people who had depression at some point in their lives were about a third more likely to suffer a stroke than those who haven't been depressed.

The analysis "seems very convincing," said Maria Glymour, a professor at the Harvard School of Public Health, who was not involved in the study.

What is not understood, she added, is whether depression is somehow causing the increased risk of stroke, or whether the two diseases have the same underlying causes.

A previous study by Glymour and her colleagues was included in the latest research, led by Dr. Li-Qiang Qin at Soochow University in China.

Qin's group compiled data from 17 studies, totaling more than 200,000 participants, that looked at whether people with depression were more likely to suffer a stroke than people who did not have symptoms of the mood disorder.

Each of the studies began with people who had not yet had a stroke, and followed them for anywhere from three to 29 years.

Two studies reported that depression was tied to a lower risk of stroke, and another two found nearly no difference in risk between those with depression and those without.

The other 13 studies showed an increased risk of stroke for people with depression, and when the team combined all 17 studies, they found that the risk of stroke was 34 percent higher among those with depression.

Stroke is the third leading cause of death in the United States. Some 800,000 Americans -- or 26 out of every 10,000 -- have a stroke each year, according to the Centers for Disease Control and Prevention.

A 34 percent increase in the number of strokes would raise that count to 35 out of every 10,000 people.

A study earlier this year, which pooled the results of 28 research projects on stroke and depression, also found an increased risk of stroke among people with depression (see Reuters Health story of September 21, 2011).

Neither study determined whether one disease is causing the other, and Glymour speculates that the association between stroke and depression is likely explained by unhealthy behaviors.

"For example, we know that depression affects the behavior that influences your stroke risk, such as diet, physical activity, adherence to health advice," Glymour told Reuters Health. "Those things take a certain amount of energy, and being depressed might affect your ability to do them."

The study authors also note that depression is linked to the development of diabetes and hypertension, which are also risk factors for stroke.

Glymour said it will be important to figure out if treating depression symptoms will lead to a decreased risk of stroke.